Dense cellular network deployments relying on the use of Massive MIMO technology are becoming very attractive candidates for future radio access technologies. This is partly due to the promise of Massive MIMO for providing very large throughput increases per BS, due to its ability to multiplex a large number of high-rate streams over each transmission resource element.
It is well accepted by now that major gains in the PHY layer in terms of throughput per unit area are to come from the judicious use of dense infrastructure antenna deployments, comprising of a dense network of small cells, possibly equipped with large antenna arrays. Indeed, Massive MIMO is very attractive when it is used over dense (small cell) deployments, as, in principle, it can translate to massive throughput increases per unit area with respect to existing deployments.
Massive MIMO is also envisioned as a candidate for addressing large variations in user load, including effectively serving user-traffic hotspots spots, such as e.g., malls or overcrowded squares. A deployment option that is considered attractive (especially) for serving user-traffic hots involves remote radio-head (RRH) systems in which a BS controls a massive set of antennas that are distributed over many locations. Current proposals for RRH systems consider only one or at most a few antennas per RRH unit. However, with bandwidth expected to become available at higher frequency bands (including in the mmWave band), it will become possible to space antenna elements far closer to one another and consider RRHs with possibly a large number of antennas per RRH unit. In principle, this would allow the network to simultaneously harvest densification and large-antenna array benefits thereby delivering large spectral efficiencies per unit area.
Channel state information (CSI) between each BS antenna and the user terminals is required in order to be able to serve multiple streams over the same transmission resources. CSI is obtained by the use of training pilots. A pilot is transmitted by one antenna and received by another in order to learn the channel between the two antennas. With massive arrays at the BS side, the preferred option for training (in terms of its training overhead) is to train in the uplink, as one pilot from a user terminal (UT) antenna trains all the antennas at nearby BS sites, no matter how many sites and antennas per site. This is true not only for transmitting data in the uplink but also for downlink transmission. By using UL training and exploiting uplink-downlink radio channel reciprocity, “Massive MIMO” rates can be achieved in the DL, provided UL training and DL massive MIMO data transmission are within the coherence time and bandwidth of the wireless propagation channel.
Furthermore, reciprocity based training inherently enables coordinated multipoint (CoMP) transmission, including RRH-based transmission. Indeed, inherently, a single pilot broadcast from a user terminal antenna trains all the antennas at all nearby BS sites that it can be received at sufficiently high power. It is well known that in cellular networks such CoMP transmission is beneficial for users at the cell edge, i.e., for users that receive equally strong signals from more than one BS. Similar performance gains are expected in RRH systems. Inherently, a user can obtain beamforming gains during the data transmission phase from all the RRH unit-antenna combinations that receive the user's pilot broadcast at sufficiently high power.
An important challenge that arises in harvesting densification benefits with cellular networks arises from the fact that UL pilot resources must be reused over the network. It is desirable to make the reuse distance of a pilot resource as small as possible in order to maximize the densification benefits and the delivered network spectral efficiency (and throughput) per unit area. Indeed, if the same pilot resource could be effectively reused by two close-by users, this would allow serving these two close-by users in parallel by the network. However, the users would have to be significantly separated (geographically), so that their simultaneously broadcasted pilots are received by their serving base-stations at sufficiently high powers, but at sufficiently low powers at each other user's BSs. This implies that there is a minimum reuse distance for a users' UL pilot that has to be honored so that users using the same pilot have to be significantly geographically separated to not cause interference to each other's BS.
A similar issue limits the throughputs per unit area achievable by RRH's. Indeed, it is conventionally assumed that a pilot resource is used by a single RRH (active) user. This limits the possible multiplexing gains offered by the RRH to serving a single user.
To achieve large cell throughputs and (especially) large cell-edge throughputs over well-planned macro-cellular networks with simplified scheduling and precoding operation, it is advocated that a reuse-7 operation is used. It is easy to show that in such a massive MIMO network, the advocated operation is effectively equivalent to a reuse-1 operation with pilot-reuse 7, whereby the pilots are split into 7 subsets and each subset is reused every 7-th cell.
There has been a proposal for a pilot reuse extension of this approach over heterogeneous networks comprising of well-planned macro-cells and small cells. In particular, pilot dimensions are split between macros and small cells. Furthermore the individual tier pilot resources are reused with a given pilot reuse factor. For example, the small cell BSs are colored with a finite set of colors, so that no small cell has a same-color neighbor. The pilot reuse factor in this case corresponds to the number of used colors. Although, in theory this results in a minimum pilot-reuse of 4, as the minimum number of needed colors is 4, in practice, larger number of colors (and thus larger pilot reuse factors) are required.
A geographic scheduling approach has been discussed, according to which, in each scheduling slot users at similar locations (relative to their serving cell) are scheduled for transmission across the network. This allows optimizing the precoder, multiplexing gains and the pilot reuse independently per geographic location, i.e., independently for cell-center and cell-edge users. With this operation, substantial gains with respect in terms of cell and cell-edge throughput (as well as in terms of the number of antennas needed to achieve a certain level of performance). However, this approach relies on a well-planned macro-cellular network with dense user traffic so that geographic scheduling and optimization are possible. As a result this approach cannot be directly used in unplanned small cell deployments.
Clearly, as higher band frequencies become available and wireless network become increasingly densified, there is a need for methods that allow translating antenna/site-densification into gains in spectral-efficiency per unit area. Although, for a well-planned macro-cellular network, this may be achieved (in this case the antenna sites remain fixed and the number of antennas per site is increased), achieving similar gains with network densification (e.g., in cases where both the number of antennas/site and the number of sites also increase) is not possible with the current state of the art methods.